1. Field of the Invention
The invention relates to a method for controlling a machine tool having adjustment means for selectively adjusting the machine tool to obtain machined pieces having different nominal dimensions within a tolerance range, comprising the step of subsequently controlling adjustments of the machine tool with respect to a plurality of nominal dimensions distributed within the tolerance range.
The invention also relates to an apparatus for controlling a machine tool for machining mechanical pieces within a determined tolerance range, comprising adjustment means for adjusting the machine tool for machining pieces having different nominal dimensions within the tolerance range, the adjustment means including setting means adapted to cause adjustments to determined nominal dimensions and control means coupled to the adjustment means.
Further, the invention relates to an apparatus for machining in series two mechanical pieces to be machined to dimensions falling within relevant tolerance ranges and to be matched to each other, comprising a first and a second machine tool, for respectively machining the two pieces, wherein each machine tool is equipped with an in-process gauge for checking the dimension of the piece being machined, the gauge having zero-setting means and control means for terminating the machining operation when the gauge reaches the zero-setting condition; a post-process gauge for checking the machined dimension of every piece and providing a relevant measurement signal; and processing means connected to the post-process gauge and the in-process gauge, for processing the measurement signal for checking the proper operation of the in-process gauge and for correcting the zero-setting means of the in-process gauge.
2. Description of the Prior Art
A machine tool for machining a type of piece according to a specific nominal dimension produces pieces whose actual dimensions may vary, with respect to the nominal dimension, in a random way, depending on intrinsic features of the machine. It is known that, in the majority of cases, the actual dimensions can be graphically represented with random variables belonging to a "normal" or Gaussian statistical distribition curve about a specific mean value. The deviation of the mean value from the nominal value and the dispersion degree (represented in statistical terms by variance .sigma..sup.2) about the value represent a characteristic of a specific machine.
A series production of pieces of two different types that are to be matched to each other gives rise to various problems. Apart from the specific tolerances allowed for each type of piece, it is also necessary to consider the "matching" tolerances, or better the identification of pairs of pieces whose dimensions not only fall within the preset tolerance range allowed for each single piece, but also enable its matching.
If it is statistically true that, by independently machining pieces of different types, that are within the tolerance limits, for each single piece it is possible to produce a corresponding piece of the other type, it is also true that, according to the production methods known up to now, this may require machining of a very large number of pieces and the formation of very big stocks.
In order to select the right pieces to be matched from two very big batches, and in order to prevent a very slow and expensive piece by piece manual search, it is necessary to classify the pieces within tolerance in various sub-classes and identify the pieces belonging to each sub-class by marking them in different ways. A method of this type, besides manifestly causing slackening, requires the use of complex apparatuses, with checking means for classifying the machined pieces in different sub-classes, and means for physically accomplishing the marking operations. This is obviously appreciably expensive.
In order to cut down the selecting time and minimize the space required for stocking the single pieces to be matched, it is generally advantageous to perform assembling (matching) of the pieces through small batches of machined pieces classified at the output of the machine.
In order to accomplish this, by employing the known apparatuses, it is necessary, for pieces of corresponding batches, that the Gaussian distribution curves of the pieces belonging to one or the other type not only be substantially within the tolerance range and have the same variance, but also be in the same position (centered with respect to corresponding mean values). If this is not true, it is practically impossible to form pairs of totally matchable batches.
Considering that the position of the Gaussian curves may vary within the tolerance range owing to various factors (for example temperature changes) and that, as already mentioned, different machines are characterized by Gaussian curves with different variances, it is easily understood how automatic machining and assemblying made "by batches" are practically unfeasible, by known systems, at least if these systems are not provided with a complex central control system.
This control system could be obtained by means of a central computer capable of simultaneously controlling the operation of the two machines, more specifically the position of the Gaussian curves (distribution curves of the machined pieces), and of detecting any displacements of each of the two distribution curves in order to command corresponding displacements of the other.
Moreover, the computer should also control proper adjustments for coping with the possible difference between the variances of the Gaussian curves.
This system is complex and costly, right for the use of a computer that has to perform a great number of processing operations with many data (for example measurement detecting, computing and updating the Gaussian curves at every new piece measured, comparing the two Gaussian curves at every measured piece, computing the displacements of the Gaussian curves, controls . . . ); furthermore it is not particularly flexible, i.e. the pieces coming out have to be immediately matched to each other or otherwise kept in the exact machining order: in fact, if they are mixed up some time before matching, the subsequent identification of the right pairs to match may become impossible.
In view of the above, a widely used method, that is known from U.S. Pat. No. 4,274,230 and German Pat. No. 1652193, consists in machining one of the two types of pieces by zero-setting the machine tool with respect to subsequent relevant pieces of the other type, previously machined. This match-machining requires very precise machine tools (at least for one type of pieces) and matching immediately or at least storing together the relevant pairs of pieces.